Field Validation of Geologic Assessment of Features Sensitive to Pollution in Karst

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2007

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Karst on Cretaceous bedrock covered by thin soils is characteristic of the Edwards aquifer recharge zone in central and south Texas, USA. Upland areas containing karst features are undergoing rapid development in the aquifer recharge zone. This study undertakes an assessment of the hydrologic function of typical small-scale upland karst features. Features included in this study were located in both the Barton Springs and San Antonio segments and the Edwards aquifer contributing zone.

Texas state law (Edwards Rules, Title 30 Texas Administrative Code (TAC) Chapter 213) reduces risks of groundwater degradation by identifying geomorphic indicators of karst "sensitive features" as the most likely areas of direct connection between the surface and the aquifer where rapid recharge can occur. At these locations, best management practices (BMPs) are required to mitigate the impacts associated with development. This project is designed to improve the effectiveness of these efforts by measuring infiltration rates of typical abundant, small upland karst features to determine if they are sensitive under the Edwards Rules.

For the test program, selected typical small upland karst features were soil-floored sinkholes, medium sinkholes with cobble-filled drains, a partially excavated solution cavity, and various background upland areas. Typical small upland karst features were paired with non-karst control plots to determine if recharge was rapid at the karst features. Constant head infiltration measurements with a large diameter single-ring infiltrometer provided direct evidence of the net hydrologic function of the contained soil and bedrock system. Microtopographical and soil thickness surveys, introduction of blue dye, followed by excavation, were used to confirm that the karst features were of karst origin with central drains through which soil was lost by sapping. Ground-penetrating radar (GPR) over tested features failed to locate significant voids in the subsurface beneath the features tested.

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